The pivotal point from which action can occur is called a catalyst. A catalyst is a substance that increases the rate of a chemical reaction without being consumed in the process. It provides the necessary activation energy for the reaction to proceed.
It is called the fulcrum.
the climax of the story
I can give you several sentences.This is a pivotal point in your career.The battle was a pivotal one, and may decide the outcome of the war.She is the pivotal player in this situation.
the turning point of a event
The Battle of Stalingrad was a pivotal point in World War II for the German army.
Its also called the turning point and the breaking point.
The fall of man
Melting occur at the melting point. The solid is transformed in a liquid.
Such mutations are called point mutation or gene mutation.
That is the climax
plot
It is called the epicenter which is the starting point.